IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communications networks (3GPP TS 22.228). IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network.
The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (UEs) or between UEs and application servers (AS). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly.
Within an IMS network, control entities, which set up and control calls or sessions, such as Call/Session Control Functions (CSCFs) operate as SIP entities within the IMS. The 3GPP architecture defines three types of CSCFs:                the Proxy CSCF (P-CSCF) which is the first point of contact within the IMS for a user terminal UE (also referred to as SIP terminal);        the Serving CSCF (S-CSCF) which provides services to the user that the user is subscribed to; and        the Interrogating CSCF (I-CSCF) whose role is to select the correct S-CSCF during registration and, in a terminating session, to find the correct S-CSCF that serves the targeted user terminal.        
IMS application services are implemented using application servers (AS). For any given public user identity, one or more application servers may be linked in by the S-CSCF on communication originated or terminated to that users UE. Application servers communicate with an S-CSCF via the IMS Service Control (ISC) interface and are linked into a SIP messaging route as required (e.g. as a result of the triggering of Initial Filter Criteria (IFC) downloaded into the S-CSCF for a given UE).
A user terminal UE registers in the IMS network using the specified SIP REGISTER method. This is a mechanism for attaching to the IMS network and announcing to the IMS network the network address at which a SIP user identity can be reached. In 3GPP, when a SIP terminal performs a registration, the IMS network authenticates the user using subscription information stored in a Home Subscriber Server (HSS), and allocates an S-CSCF to that user from the set of available S-CSCFs. Whilst the criteria for allocating S-CSCFs are not specified by 3GPP, these may include load sharing and service requirements. It is noted that the allocation of an S-CSCF is key to controlling, and charging for, user access to IMS-based services. Operators may provide a mechanism for preventing direct user-to-user SIP sessions which would otherwise bypass the S-CSCF.
During the registration process, it is the responsibility of the I-CSCF to select an S-CSCF, if an S-CSCF is not already selected. The I-CSCF receives the required S-CSCF capabilities from the HSS, and selects an appropriate S-CSCF based on the received capabilities. It is noted that S-CSCF allocation is also carried out for a user by the I-CSCF in the case where the user is called by another party, and the user is not currently allocated an S-CSCF. When a registered user subsequently sends a session request to the IMS network, the P-CSCF is able to forward the request to the selected S-CSCF based on information received from the S-CSCF during the registration process.
3GPP has defined the IMS Communication Service Identifier (ICSI) to explicitly identify the type of communication service that is to be established over the IMS network. The ICSI may be incorporated in a request initiating a session. The IMS network elements and the terminating user terminal UE need to have the same understanding of what communication service is requested. One example is to separate the two incompatible IMS implemented “voice” services from each other, PoC (Push-to-talk) and Multimedia telephony. The ICSI is used also for many other purposes such as:
Linking in Application Servers over IMS Service Control (ISC),
Discrimination in end-points to the correct software module,
Policy handling and enforcement, and
Interconnect between operators.
The IMS Communication Service Identifier (ICSI) has been included in 3GPP.
A problem may occur when a user terminal UE connects to an IMS network without including an ICSI which may lead to several nodes having to “guess” what communication service to execute their service logic for, see for example “3rd generation Partnership project; Technical Specification Group Services and System Aspects: identification of Communication Services in IMS (Release 7), 3GPP TS 23.816 version 7.0.0.
As described above, the reason why a user terminal UE may not include an ICSI may e.g. be that the user terminal DE is a legacy terminal that does not support the IMS Communication Service Identifier. However, it is also possible that the user terminal UE generates an IMS Communication Service Identifier that does not conform to any of the communication services defined in the IMS network and is not recognized by the IMS network. For both examples there is no appropriate way of dealing with such requests.
It is an object to enable processing of requests for sessions that do not properly specify the kind of service that is requested.